ok this may sound crazy but when both windows are fully down and i keep my hand on the window switches, the exhaust note changes and the engine seems to idle differently and as soon as i let go it does a kind of rev ? any ideas ?

extra load on the alternator to maintain voltage causing load on the engine hence revs dropping

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Will

so you think it could be an alternator fault ? if so is it easy to change ?

no its no fault just about all cars do it mate

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Alot of cars do it

As said, you're putting extra load on the alternator so the engine has to work harder to keep it's idle. This is why they say that having your radio, heaters, rear window heater and everything else on in you car causes your fuel economy to go down

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